DUBAI, United Arab Emirates (AP) — A Saudi-led coalition backing Yemen’s exiled government began an assault Wednesday on the port city of Hodeida, the main entry for food into a country already on the brink of famine, raising warnings from aid agencies that Yemen’s humanitarian disaster could deepen.

The assault on the Red Sea port aims to drive out Iranian-aligned Shiite rebels known as Houthis and their allies, who have held Hodeida since 2015, and a victory could be a major shift in a war that has been stalemated. But it could bring the first major street-to-street fighting for the coalition, a potentially dragged out battle deadly for combatants and civilians alike.

The fear is that a protracted fight could force a shutdown of Hodeida’s port at a time when a halt in aid risks tipping millions into starvation. Some 70 percent of Yemen’s food enters the country via the port, as well as the bulk of humanitarian aid and fuel supplies. Around two-thirds of the country’s population of 27 million relies on aid and 8.4 million are even worse off, at risk of starving already.

Before dawn Wednesday, convoys of vehicles appeared to be heading toward the rebel-held city, according to videos posted on social media. The sound of heavy, sustained gunfire clearly could be heard in the background.

Saudi-owned satellite news channels and later state media announced the battle had begun, citing military sources. They also reported coalition airstrikes and shelling by naval ships.

The initial battle plan appeared to involve a pincer movement. Some 2,000 troops who crossed the Red Sea from an Emirati naval base in the African nation of Eritrea landed west of the city with plans to seize Hodeida’s port, Yemeni security officials said.

Emirati forces with Yemeni troops moved in from the south near Hodeida’s airport, while others sought to cut off Houthi supply lines to the east, the officials said. They spoke on condition of anonymity as they weren’t authorized to brief journalists.

Yemen’s exiled government “has exhausted all peaceful and political means to remove the Houthi militia from the port of Hodeida,” it said in a statement. “Liberation of the port of Hodeida is a milestone in our struggle to regain Yemen from the militias.”

“The targeted ship was carrying troops prepared for a landing on the coast of Hodeida,” the channel said.

The Saudi-led coalition did not immediately acknowledge the incident. The U.S. Navy’s 5th Fleet, whose area of responsibility includes the Red Sea, referred questions to the Pentagon, which did not immediately respond to a request for comment.

Forces loyal to Yemen’s exiled government and irregular fighters led by Emirati troops had neared Hodeida in recent days. The port is some 150 kilometers (90 miles) southwest of Sanaa, Yemen’s capital held by the Houthis since they swept into the city in September 2014. The Saudi-led coalition entered the war in March 2015 and has received logistical support from the U.S.

Emirati Minister of State for Foreign Affairs Anwar Gargash earlier told French newspaper Le Figaro the deadline for a withdrawal from Hodeida by the Houthis expired early Wednesday morning.

The United Nations and other aid groups already had pulled their international staff from Hodeida ahead of the rumored assault.

However, so far, the port remains open, with supplies arriving. Several ships arrived in the past days, including oil tankers, and there has been no word from the coalition or U.N. to stop work, according to a senior port official, who spoke on condition of anonymity because he was not authorized to talk to the press.

“If this vital route for supplying food, fuel and medicine is blocked, the result will be more hunger, more people without health care and more families burying their loved ones,” Oxfam’s country director in Yemen, Muhsin Siddiquey, warned last week.

Over 10,000 people have been killed in Yemen’s civil war, which has displaced 2 million more and helped spawn a cholera epidemic. The Saudi-led coalition has been criticized for its airstrikes killing civilians. Meanwhile, the U.N. and Western nations say Iran has supplied the Houthis with weapons from assault rifles up to the ballistic missiles they have fired deep into Saudi Arabia, including at the capital, Riyadh.

The war has also pushed Yemen into near famine. The coalition has blockaded most ports, letting supplies into Hodeida in coordination with the U.N. A Saudi-led airstrike in 2015 destroyed cranes at Hodeida. The United Nations in January shipped in mobile cranes to help unload ships there. The air campaign and fighting has also disrupted supply lines and caused an economic crisis that made food too expensive for many to buy.

The U.N. says some 600,000 people live in and around Hodeida, and “as many as 250,000 people may lose everything - even their lives” in the assault. Already, Yemeni security officials said some were fleeing the fighting.

“We hear sounds of explosions. We are concerned about missiles and shells. Some workers have left to their villages for fear of the war,” said Mohammed, a Hodeida resident who gave only his first name for fear of reprisals.

Aid workers had similar fears.

“We have had more than 30 airstrikes within 30 minutes this morning around the city. Some civilians are entrapped, others forced from their homes,” said Jolien Veldwijk, the acting country director of the aid group CARE International, which works in Hodeida. “We thought it could not get any worse, but unfortunately we were wrong.”

U.N. Secretary-General Antonio Guterres had said that U.N. envoy Martin Griffiths was in “intense negotiations” in an attempt to avoid a military confrontation. However, Griffiths’ recent appointment as envoy and his push for new negotiations may have encouraged the Saudi-led coalition to strengthen its hand ahead of any peace talks with the Houthis.

The attack also comes as Washington has been focused on President Donald Trump’s recent summit with North Korean leader Kim Jong Un. A statement from Secretary of State Mike Pompeo said Monday he spoke with Emirati officials and “made clear our desire to address their security concerns while preserving the free flow of humanitarian aid and life-saving commercial imports.”

Defense Secretary Jim Mattis on Monday acknowledged the U.S. continues to provide support to the Saudi-led coalition.

“It’s providing any intel, or anything we can give to show no-fire areas where there are civilians, where there’s mosques, hospitals, that sort of thing - (and) aerial refueling, so nobody feels like I’ve got to drop the bomb and get back now,” he said.

It wasn’t immediately clear what specific American support the coalition was receiving Wednesday.